The present invention relates to a method for fabricating a semiconductor device and more particularly, to a method for fabricating an isolation layer in a semiconductor device.
In fabricating a semiconductor device, a plurality of elements formed on a semiconductor wafer must be isolated from each other. As one of the methods for isolating the elements from each other, the local oxidation of silicon (LOCOS) process is widely known and used.
Among the different LOGOS processes, the poly-buffer-recessed (PBR) LOCOS process will be explained below with reference to FIGS. 1A, 1B and 1C.
Referring to FIG. 1A, a pad oxide layer 11 is formed on a semiconductor substrate 10, and a polysilicon layer 12 and a CVD (chemical vapor deposition) nitride layer 13 are sequentially formed thereon. Then the CVD nitride layer 13, polysilicon layer 12 and pad oxide layer 11, which are placed on a field region of the substrate 10, are sequentially dry-etched using a LOCOS mask. The LOCOS mask defines a portion of the substrate on which a LOCOS oxide layer is to be formed. The surface of the substrate 10 exposed by dry etching may be further etched. However, it is possible to omit the further etching of the substrate 10.
Referring to FIG. 1B, a thin oxide layer 14 is formed on the exposed field region. Then a CVD (chemical vapor deposition) nitride layer is formed and etched back to form a nitride layer 15 on the side of the oxide layer 14. Here, the thin oxide layer 14 is formed according to the natural oxidation or thermal oxidation of the exposed portion of the substrate 10 and polysilicon layer 12.
When the nitride layer 15 is formed with etch back, the portion of the nitride layer 15 formed on the substrate of the field region is removed, and the portion of the nitride layer 15 formed on the side of the pad oxide layer 11 and polysilicon layer 12 is maintained, thereby forming the (sidewall) nitride layer 15. Then, a field oxidation is carried out to form a field oxide layer 16 as shown in FIG. 1C. Thereafter, the nitride layers 13 and 15, polysilicon layer 12 and pad oxide layer 11 are removed so that only the field oxide layer 16 remains on the substrate 10.
The aforementioned conventional LOCOS process, however, requires the sidewall nitride layer 15 to be deposited through CVD (chemical vapor deposition) and etched back by dry etching. During dry etching of the nitride layer 15 the oxide layer 14 is etched unintentionally since the oxide layer 14 is relatively thin. In some instances, even the substrate 10 may be etched unintentionally. Therefore, the conventional LOCOS process requires accurate control of etch rate of the nitride layer 15 which is difficult to obtain. Consequently, the conventional process for forming the sidewall nitride layer is complicated.
Furthermore, after the formation of the sidewall nitride layer 15, the nitride layer obtains a gentle slope. In case that the field region is defined by the size less than 0.3 .mu.m, a sufficient space in which the field oxide layer is to be formed cannot be secured. But, even if the sufficient space is secured, a large bird's beak is formed in the remaining space which results in an insufficient space for isolating the semiconductor elements from one another.